Nitride semiconductor device using insulating films having different bandgaps to enhance performance

ABSTRACT

The semiconductor device includes: a channel layer, a barrier layer, a first insulating film, and a second insulating film, each of which is formed above a substrate; a trench that penetrates the second insulating film, the first insulating film, and the barrier layer to reach the middle of the channel layer; and a gate electrode arranged in the trench and over the second insulating film via a gate insulating film. The bandgap of the second insulating film is smaller than that of the first insulating film, and the bandgap of the second insulating film is smaller than that of the gate insulating film GI. Accordingly, a charge (electron) can be accumulated in the second (upper) insulating film, thereby allowing the electric field strength at a corner of the trench to be improved. As a result, a channel is fully formed even at a corner of the trench, thereby allowing an ON-resistance to be reduced and an ON-current to be increased.

CROSS-REFERENCE TO RELATED APPLICATIONS

The disclosure of Japanese Patent Application No. 2014-170330 filed onAug. 25, 2014 including the specification, drawings and abstract isincorporated herein by reference in its entirety.

BACKGROUND

The present invention relates to a semiconductor device and amanufacturing method of a semiconductor device, and can be preferablyused in a semiconductor device using, for example, a nitridesemiconductor and a manufacturing method thereof.

In recent years, semiconductor devices using group III-V compounds, eachof which has a bandgap larger than that of Si, have attracted attention.Among them, semiconductor devices using gallium nitride (GaN) are beingdeveloped, because gallium nitride is a material having advantagesthat: 1) the breakdown electric field is large; 2) the electronsaturation velocity is large; 3) the thermal conductivity is large; 4) agood heterojunction can be formed between AlGaN and GaN; 5) galliumnitride is non-poisonous and highly safe; and the like.

Further, semiconductor devices, each of which is a power MISFET (MetalInsulator Semiconductor Field Effect Transistor) using gallium nitridebecause of its high withstand voltage and a high-speed switchingcharacteristic and in each of which a normally-off operation can beperformed, are being developed.

For example, Japanese Unexamined Patent Application Publication No.2013-118343 discloses a MIS-type compound semiconductor device adoptinga gate recess structure. In this semiconductor device, a recess for agate electrode is formed in an interlayer insulating film, a passivationfilm, and a compound semiconductor lamination.

SUMMARY

The present inventors are engaged in the research and development of asemiconductor device using the aforementioned nitride semiconductor, andare intensively studying to improve the characteristics of anormally-off type semiconductor device. In the course of the researchand development, it has revealed that there is room for furtherimprovement in the characteristics of a semiconductor device using anitride semiconductor.

Other problems and new characteristics will become clear from thedescription and accompanying drawings of the present specification.

Of the preferred embodiments disclosed in the present application,outlines of the typical ones will be briefly described as follows.

A semiconductor device according to one embodiment disclosed in thepresent application has both a trench, which penetrates an upperinsulating film, a lower insulating film, and a barrier layer to reachthe middle of a channel layer, and a gate electrode arranged in thetrench and over the upper insulating film via a gate insulating film.The bandgap of the upper insulating film is smaller than that of thelower insulating film. Additionally, the bandgap of the upper insulatingfilm is smaller than that of the gate insulating film.

In a semiconductor device according to a typical embodiment that isdisclosed in the present application and described below, thecharacteristics of the semiconductor device can be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view illustrating a configuration of asemiconductor device according to First Embodiment;

FIG. 2 is a plan view illustrating the configuration of thesemiconductor device according to First Embodiment;

FIG. 3 is a sectional view illustrating a manufacturing step of thesemiconductor device according to First Embodiment;

FIG. 4 is a sectional view illustrating a manufacturing step of thesemiconductor device according to First Embodiment, the manufacturingstep following FIG. 3;

FIG. 5 is a sectional view illustrating a manufacturing step of thesemiconductor device according to First Embodiment, the manufacturingstep following FIG. 4;

FIG. 6 is a sectional view illustrating a manufacturing step of thesemiconductor device according to First Embodiment, the manufacturingstep following FIG. 5;

FIG. 7 is a sectional view illustrating a manufacturing step of thesemiconductor device according to First Embodiment, the manufacturingstep following FIG. 6;

FIG. 8 is a sectional view illustrating a manufacturing step of thesemiconductor device according to First Embodiment, the manufacturingstep following FIG. 7;

FIG. 9 is a sectional view illustrating a manufacturing step of thesemiconductor device according to First Embodiment, the manufacturingstep following FIG. 8;

FIG. 10 is a sectional view illustrating a manufacturing step of thesemiconductor device according to First Embodiment, the manufacturingstep following FIG. 9;

FIG. 11 is a sectional view illustrating a manufacturing step of thesemiconductor device according to First Embodiment, the manufacturingstep following FIG. 10;

FIG. 12 is a sectional view illustrating a manufacturing step of thesemiconductor device according to First Embodiment, the manufacturingstep following FIG. 11;

FIG. 13 is a sectional view illustrating a manufacturing step of thesemiconductor device according to First Embodiment, the manufacturingstep following FIG. 12;

FIGS. 14A to 14D are schematic views of the vicinity of an end portionof the bottom surface of a trench, these views presented for explainingeffects in First Embodiment;

FIGS. 15A to 15D are band diagrams illustrating situations of chargeinjection;

FIG. 16 is a timing chart illustrating both a step of injecting a chargeinto an upper insulating film in a transistor and a driving step of thetransistor;

FIG. 17 is a sectional view illustrating a configuration of asemiconductor device according to Second Embodiment;

FIG. 18 is a plan view illustrating the configuration of thesemiconductor device according to Second Embodiment;

FIG. 19 is a sectional view illustrating the configuration of thesemiconductor device according to Second Embodiment;

FIG. 20 is a sectional view illustrating a manufacturing step of thesemiconductor device according to Second Embodiment;

FIG. 21 is a sectional view illustrating a manufacturing step of thesemiconductor device according to Second Embodiment, the manufacturingstep following FIG. 20;

FIG. 22 is a sectional view illustrating a manufacturing step of thesemiconductor device according to Second Embodiment, the manufacturingstep following FIG. 21;

FIG. 23 is a sectional view illustrating a manufacturing step of thesemiconductor device according to Second Embodiment, the manufacturingstep following FIG. 22;

FIG. 24 is a sectional view illustrating a manufacturing step of thesemiconductor device according to Second Embodiment, the manufacturingstep following FIG. 23;

FIG. 25 is a sectional view illustrating a manufacturing step of thesemiconductor device according to Second Embodiment;

FIG. 26 is a plan view illustrating a manufacturing step of thesemiconductor device according to Second Embodiment;

FIG. 27 is a sectional view illustrating a manufacturing step of thesemiconductor device according to Second Embodiment, the manufacturingstep following FIG. 24;

FIG. 28 is a sectional view illustrating a manufacturing step of thesemiconductor device according to Second Embodiment, and accords withline B designated in FIG. 18;

FIG. 29 is a plan view illustrating a manufacturing step of thesemiconductor device according to Second Embodiment;

FIG. 30 is a sectional view illustrating a manufacturing step of thesemiconductor device according to Second Embodiment, the manufacturingstep following FIG. 27;

FIG. 31 is a sectional view illustrating a manufacturing step of thesemiconductor device according to Second Embodiment, and accords withline B designated in FIG. 18;

FIGS. 32A and 32B are schematic views of the vicinity of an end portionof the bottom surface of a trench, these views presented for explainingeffects in Second Embodiment; and

FIG. 33 is a timing chart illustrating both a step of injecting a chargeinto an upper insulating film in a transistor and a driving step of thetransistor.

DETAILED DESCRIPTION

If needed for convenience, the following embodiments will be describedby dividing each of them into multiple sections or embodiments; however,the multiple sections or embodiments are not irrelevant to each other,but they are in a relationship in which one is a variation, applicationexample, detailed description, or supplementary description of part orthe whole of the others, unless otherwise indicated. When the numbers ofelements, etc. (including numbers of pieces, numerical values, amounts,ranges, etc.) are referred to in the following embodiments, the numbersare not limited to the specific ones but may be more or less than thespecific numbers, unless otherwise indicated or except when the numbersare obviously limited to the specific numbers in principle.

Further, in the following embodiments, the constituents (also includingelement steps, etc.) are not necessarily essential, unless otherwiseindicated or clearly essential in principle. Similarly, when the shapesand positional relations, etc., of the constituents, etc., are referredto in the following embodiments, those substantially the same or similarto the shapes, etc., should also be included, unless otherwise indicatedor except when considered to be clearly otherwise in principle. The sameis true with the aforementioned numbers, etc., (including the numbers ofpieces, numerical values, amounts, and ranges, etc.).

Hereinafter, preferred embodiments will be described in detail withreference to the accompanying views. In the whole views for explainingthe embodiments, members having the same function as each other will bedenoted with the same or relevant reference numeral and duplicativedescription will be omitted. When a plurality of similar members (parts)are present, an individual or specific part may be represented by addinga sign to the collective reference numeral. In the followingembodiments, description of the same or similar parts will not berepeated in principle, unless particularly necessary.

In the views used in the embodiments, hatching may be omitted even insectional views in order to make them easier to see.

In a sectional view or a plan view, the size of each part does notcorrespond to that of an actual device, and a specific part may bedisplayed to be relatively large in order to make the view easier tounderstand. The same is true with the case where a sectional view and aplan view correspond to each other.

First Embodiment

Hereinafter, a semiconductor device according to the present embodimentwill be described in detail with reference to the views.

[Structure Description]

FIG. 1 is a sectional view illustrating a configuration of thesemiconductor device according to the present embodiment. FIG. 2 is aplan view illustrating the configuration of the semiconductor deviceaccording to the embodiment. The sectional view of FIG. 1 corresponds,for example, to A-A Section in FIG. 2.

The semiconductor device according to the present embodiment is a MIS(Metal Insulator Semiconductor) type FET (Field Effect Transistor) usinga nitride semiconductor. This semiconductor device is also referred toas an HEMT (High Electron Mobility Transistor) or a power transistor.The semiconductor device according to the embodiment is a so-calledrecess gate-type semiconductor device.

In the semiconductor device according to the present embodiment, achannel layer CH and a barrier layer BA are formed in this order over asubstrate S, as illustrated in FIG. 1. An active region AC, in which atransistor is to be formed, is divided by an element isolation regionISO (see FIG. 2).

An insulating film (IF1, IF2) is formed over the barrier layer BA. Thisinsulating film includes two layers of insulating films. A lowerinsulating film IF1 is formed over the barrier layer BA, and an upperinsulating film IF2 is formed over the lower insulating film IF1.

This insulating film (IF1, IF2) has a role of an etching stopper when agate electrode GE is patterned. The upper insulating film IF2 is a filmhaving a bandgap smaller than that of the lower insulating film IF1.Additionally, the upper insulating film IF2 is a film having a bandgapsmaller than that of the later-described gate insulating film GI (seeFIG. 15).

The gate electrode GE is formed, via the gate insulating film GI, in atrench T that penetrates the insulating film (IF1, IF2) and the barrierlayer BA to reach the middle of the channel layer CH. The channel layerCH and the barrier layer BA include a nitride semiconductor, and thebarrier layer BA is a nitride semiconductor having a bandgap wider thanthat of the channel layer CH.

A two-dimensional electron gas 2DEG is generated in the vicinity of theinterface between the channel layer CH and the barrier layer BA, thevicinity being near to the channel layer CH. When a threshold potential(potential V2>0, also referred to as a drive potential) is applied tothe gate electrode GE, a channel is formed in the vicinity of theinterface between the gate insulating film GI and the channel layer CH.

The two-dimensional electron gas 2DEG is formed by the followingmechanism. The nitride semiconductors (herein, gallium nitride-basedsemiconductors), by which the channel layer CH and the barrier layer BAare formed, are different from each other in bandgap and electronaffinity. Thereby, a square-well potential is generated in a junctionsurface between these semiconductors. The two-dimensional electron gas2DEG is generated in the vicinity of the interface between the channellayer CH and the barrier layer BA by accumulating an electron in thesquare-well potential.

Herein, the two-dimensional electron gas 2DEG formed in the vicinity ofthe interface between the channel layer CH and the barrier layer BA isdivided by the trench T in which the gate electrode GE is formed.Therefore, in the semiconductor device according to the presentembodiment, an OFF state can be maintained while the threshold potentialis not being applied to the gate electrode GE, and an ON state can bemaintained with a channel being formed while the threshold potential isbeing applied thereto. Thus, a normally-off operation can be performed.

The configuration of the semiconductor device according to the presentembodiment will be further described in detail. In the semiconductordevice according to the embodiment, the channel layer CH including anitride semiconductor is formed over the substrate S, and the barrierlayer BA including a nitride semiconductor is formed over the channellayer CH, as illustrated in FIG. 1. Alternatively, a nucleation layer, astrain relaxation layer, and a buffer layer, etc., may be providedbetween the substrate S and the channel layer CH in this order from thesubstrate S side. These layers include a nitride semiconductor. Thenucleation layer is formed in order to produce a crystal nucleus when alayer to be formed above, such as the strain relaxation layer, isgrowing. Additionally, the nucleation layer is formed in order toprevent the substrate S from deteriorating with the constituent of alayer to be formed above (e.g., Ga, etc.) diffusing from the layer tothe substrate S. The strain relaxation layer is formed in order tosuppress occurrence of a warp or crack in the substrate S by relaxing astress to be applied to the substrate S. The buffer layer is anintermediate layer located between the channel layer CH and the strainrelaxation layer.

The gate electrode GE is formed, via the gate insulating film GI, in thetrench (also referred to as a recess) T that penetrates the insulatingfilm (IF1, IF2) and the barrier layer BA and is dug to the middle of thechannel layer CH.

The insulating film (IF1, IF2) has an opening in an opening region (OA1)(see FIG. 5). The trench T is formed to correspond to the opening.

The gate insulating film GI is formed in the trench T and over theinsulating film (IF1, IF2). The gate electrode GE is formed over thegate insulating film GI. The shape of the gate electrode GE obtainedwhen viewed from above (hereinafter, the shape is referred to as aplanar shape) is, for example, rectangular (see FIG. 2). Herein, theplanar shapes of the gate insulating film GI and the gate electrode GEare the same as each other.

The gate electrode GE has a shape protruding in one direction (to theright side, i.e., to a drain electrode DE side in FIG. 1). Theprotruding part is referred to as a field plate electrode (also referredto as a Faraday shield electrode). The field plate electrode is apartial region of the gate electrode GE extending from the end portionof the trench T near to the drain electrode DE toward the drainelectrode DE.

The gate electrode GE also extends from the end portion of the trench Tnear to a source electrode SE toward the source electrode SE. Theinsulating film (IF1, IF2) is arranged below the gate electrodeprotruding (extending) toward the drain electrode DE or the sourceelectrode SE.

Additionally, the source electrode SE and the drain electrode DE areformed over the barrier layer BA on both the sides of the gate electrodeGE. The barrier layer BA and the source electrode SE are ohmic-coupledtogether via an ohmic layer. The barrier layer BA and the drainelectrode DE are ohmic-coupled together via an ohmic layer. The sourceelectrode SE includes both a coupling part (plug), located in a contacthole C1S formed in an interlayer insulating film IL1, and a wiring partlocated over the coupling part. The drain electrode DE includes both acoupling part (plug), located in a contact hole C1D formed in theinterlayer insulating film IL1, and a wiring part located over thecoupling part. The source electrode SE and the drain electrode DE arecovered with a protective insulating film PRO. The planar shape of eachof the source electrode SE and the drain electrode DE is, for example,rectangular (see FIG. 2).

The aforementioned gate electrode GE is coupled to gate wiring GL via acoupling part (plug) located in a contact hole C1G formed in theinterlayer insulating film IL1 (see FIG. 2).

By thus forming the upper insulating film IF2 with a film having abandgap smaller than that of the lower insulating film IF1, a charge(herein, electron) can be accumulated in the upper insulating film IF2,as described later, thereby allowing the electric field strength at acorner of the trench to be improved. As a result, a channel is fullyformed even at the corner of the trench, thereby allowing anON-resistance to be reduced and an ON-current to be increased. Thus, thedriving capability of the transistor can be improved.

[Description of Manufacturing Method]

Subsequently, a manufacturing method of the semiconductor deviceaccording to the present embodiment will be described with reference toFIGS. 3 to 13, so that the configuration of the semiconductor devicewill be made clearer. FIGS. 3 to 13 are sectional views eachillustrating a manufacturing step of the semiconductor device accordingto the embodiment.

The channel layer CH is formed over the substrate S, as illustrated inFIG. 3. For example, a semiconductor substrate including silicon (Si)that has a resistivity of 1 Ω·cm and the (111) surface of which isexposed is used as the substrate S, and a gallium nitride (GaN) layer isheteroepitaxially grown thereover as the channel layer CH by using anMOCVD (Metal Organic Chemical Vapor Deposition) method, or the like. Thethickness of the channel layer CH is, for example, approximately 1 μm.Alternatively, a substrate including Sic or sapphire, other than thesilicon, may be used as the substrate S. Additionally, a nucleationlayer, a strain relaxation layer, and a buffer layer may be providedbetween the substrate S and the channel layer CH in this order from thesubstrate S side. For example, an aluminum nitride (AlN) layer is usedas the nucleation layer; a laminated film (AlN/GaN film) of a galliumnitride (GaN) layer and an aluminum nitride (AlN) layer is used as thestrain relaxation layer; and an AlGaN layer, or the like, is used as thebuffer layer. These layers can be formed by using an MOCVD method, orthe like. In this case, the thickness of a portion ranging from thesurface of the substrate S to the surface of the channel layer CH isapproximately 3 to 5 μm.

Subsequently, for example, an AlGaN (Al_(X)Ga_((1-X))N) layer isheteroepitaxially grown over the channel layer CH as the barrier layerBA by using an MOCVD method, or the like. The thickness of the AlGaNlayer is, for example, approximately 0.03 μm.

Thus, a laminated body of the channel layer CH and the barrier layer BAis formed. The laminated body is formed by the aforementionedheteroepitaxial growth, i.e., by group-III-face growth in whichlamination is performed in the [0001] crystal axis (C-axis) direction.In other words, the laminated body is formed by the (0001) Ga-facegrowth. The two-dimensional electron gas 2DEG is generated in thevicinity of the interface between the channel layer CH and the barrierlayer BA, in the laminated body.

Subsequently, the insulating film (IF1, IF2) is formed over the barrierlayer BA as a cover film. For example, a silicon oxide film (SiO₂ film,film including silicon oxide) is deposited, as the insulating film IF1,over the barrier layer BA by using a CVD (Chemical Vapor Deposition)method, or the like, so as to have a thickness of approximately 0.02 μm.Subsequently, a silicon nitride film (Si₃N₄ film, film including siliconnitride) is deposited, as the insulating film IF2, over the insulatingfilm IF1 by using a CVD method, or the like, so as to have a thicknessof approximately 0.02 μm. The bandgap of the lower silicon oxide film isapproximately 8.9 eV, while that of the upper silicon nitride film isapproximately 4.5 eV. Thus, the bandgap of the upper silicon nitridefilm is smaller than that of the lower silicon oxide film (see FIG. 15).

Subsequently, a photoresist film PR1 for opening an element isolationregion is formed over the insulating film (IF1, IF2) by using aphotolithography technique, as illustrated in FIG. 4. Subsequently,boron (B) or nitrogen (N) is doped by using the photoresist film PR1 asa mask. The boron (B) or nitrogen (N) is injected into the channel layerCH and the barrier layer BA via the insulating film (IF1, IF2). By thusdoping an ion species, such as boron (B), nitrogen (N), or the like,into the channel layer CH and the barrier layer BA, a crystal state ischanged to have a higher resistance. The element isolation region ISO isthus formed. Thereafter, the photoresist film PR1 is removed. The regionsurrounded by the element isolation region ISO will serve as the activeregion AC (see FIG. 2).

Subsequently, a photoresist film PR2 having an opening in an openingregion OA1 is formed over the insulating film IF2 by using aphotolithography technique, as illustrated in FIG. 5. Subsequently, theinsulating film (IF1, IF2) is etched by using the photoresist film PR2as a mask. Herein, a process, in which a material of lower layer is thusprocessed into a desired shape by performing etching with the use of aphotoresist film processed into a desired shape by photolithography(exposure, development) or a hard mask as a mask, is referred to aspatterning. A gas, such as, for example, CF₄ or CHF₃, can be used as theetching gas for the silicon nitride film and the silicon oxide film.Thereby, the insulating film (IF1, IF2) having an opening in the openingregion OA1 is formed over the barrier layer BA. In other words, thebarrier layer BA is exposed in the opening region OA1 (FIG. 5).

Continuously, the trench T, which penetrates the insulating film (IF1,IF2) and the barrier layer BA to reach the middle of the channel layerCH, is formed by etching the barrier layer BA and the channel layer CHwith the use of the photoresist film PR2 as a mask, as illustrated inFIG. 6. Dry etching is performed under plasma atmosphere by using, forexample, a halogen-based gas (Cl₂, HBr, BCl₃, or the like) as theetching gas. For example, ICP (Inductively Coupled Plasma), or the like,can be used as the plasma source. When the barrier layer (AlGaN) BAhaving a thickness of 0.03 ρm is used, the etching is performed to thedepth of approximately 0.04 μm from the surface of the barrier layer(AlGaN) BA in order to surely remove the two-dimensional electron gas2DEG in the opening region OA1. In other words, the level differencebetween the bottom surface of the barrier layer (AlGaN) BA and that ofthe trench T is approximately 0.01 μm. Thereby, the barrier layer(AlGaN) BA and the surface portion of the channel layer (GaN) CH in theopening region OA1 are removed, so that the channel layer (GaN) CH isexposed from the opening region OA1. Subsequently, the photoresist filmPR2 is removed.

Subsequently, the gate insulating film GI is formed in the trench T andover the insulating film (IF1, IF2), as illustrated in FIG. 7. Forexample, an aluminum oxide film (alumina, Al₂O₃) having a thickness ofapproximately 0.1 μm is deposited, as the gate insulating film GI, inthe trench T and over the insulating film (IF1, IF2) by using an ALD(Atomic Layer Deposition) method, or the like. The bandgap of the gateinsulating film GI is larger than that of the insulating film (siliconnitride film) IF2 located therebelow (see FIG. 15). The bandgap of thealuminum oxide film is approximately 6 eV, while that of the siliconnitride film located therebelow is approximately 4.5 eV.

As the gate insulating film GI, for example, a silicon oxide film or ahigh dielectric constant film having a dielectric constant higher thanthat of a silicon oxide film, other than the aforementioned aluminumoxide film, may be used. As the high dielectric constant film, otherhafnium-based insulating films, such as a hafnium oxide film (HfO₂film), a hafnium aluminate film, a HfON film (hafnium oxynitride film),a HfSiO film (hafnium silicate film), a HfSiON film (hafnium siliconoxynitride film), and a HfAlO film, may be used. The bandgap of each ofsuch hafnium-based insulating films is larger than that of a siliconnitride film.

The type and the thickness of the gate insulating film GI are designedin consideration of the operating voltage, the reliability, and thebreakdown voltage, etc., that are said to be necessary in a circuitoperation. For example, when an aluminum oxide film or a silicon oxidefilm is used as the gate insulating film GI, long-term reliability thatis practically sufficient is obtained by designing it to be used in anelectric field of 2 to 4 MV/cm. Therefore, when a transistor is designedto operate at approximately 20 to 40 V, the thickness of the gateinsulating film (aluminum oxide film or silicon oxide film) GI isrequired to be approximately 0.1 μm.

The thickness (0.1 μm) of the gate insulating film GI is mostly largerthan the depth of the trench T. Herein, the depth of the trench isdefined, as a first depth, by the level difference between the surfaceof the barrier layer BA and the bottom surface of the trench T (0.04 μmin the present embodiment). Additionally, the depth of the trench isdefined, as a second depth, by the level difference between the surfaceof the upper insulating film IF2 and the bottom surface of the trench T(0.08 μm in the embodiment). When the thickness of the gate insulatingfilm GI is 0.1 μm in the embodiment, the thickness of the gateinsulating film is larger than the first depth and the second depth.

Subsequently, a conductive film to serve as the gate electrode GE isformed over the gate insulating film GI. For example, a laminated film(also referred to as an Au/Ni film) including, for example, a nickel(Ni) film and a gold (Au) film located thereabove is deposited, as theconductive film, over the gate insulating film GI by using a sputteringmethod, or the like.

Subsequently, the gate electrode GE is formed by patterning the gateelectrode GE and the gate insulating film GI by using a photolithographytechnique and an etching technique, as illustrated in FIG. 8. Aphotoresist film PR3, covering the region where the gate electrode GE isformed, is formed by using, for example, a photolithography technique,so that the gate electrode GE and the gate insulating film GI are etchedby using the photoresist film PR3 as a mask. Dry etching is performedunder plasma atmosphere by using, for example, a halogen-based gas (Cl₂,HBr, or a mixed gas thereof) as the etching gas. For example, ICP(Inductively Coupled Plasma), or the like, can be used as the plasmasource. Thereafter, the photoresist film PR3 is removed.

The insulating film (IF1, IF2) serves as an etching stopper during theetching. If dry etching is performed on the gate electrode GE and thegate insulating film GI that are directly formed over the barrier layerBA without forming the insulating film (IF1, IF2) by using thephotoresist film PR3 as a mask, the barrier layer BA may be damaged. Inparticular, if the processing is performed under plasma atmosphere,plasma damage may be caused. Because of such damage, the two-dimensionalelectron gas is not well formed. Further, for example, if the thicknessof the barrier layer BA is large, the crystalline property may bedeteriorated and a rearrangement not preferred for a device operationmay be remarkably generated, and conversely, when the thickness thereofis small, the concentration of the two-dimensional electron gas isdecreased. Accordingly, it is preferable to form the barrier layer BA tohave an appropriate thickness (e.g., thickness of approximately 0.02 to0.04 μm). If the thickness of the barrier layer BA is thus small, thereis the fear that the barrier layer BA may be partially etched whenexposed to etching atmosphere, which causes the variation in thethickness to be large, and further the barrier layer may be partiallylost. In such a case, the two-dimensional electron gas is not wellformed, and for example, the resistance of the two-dimensional electrongas is increased. Thus, the operating characteristic of the transistoris deteriorated.

On the other hand, the insulating film (IF1, IF2) functions as anetching stopper in the present embodiment, and hence the two-dimensionalelectron gas is well formed and the characteristics of the transistorcan be improved.

Because the insulating film (IF1, IF2) functions as an etching stopper,the surface of the insulating film (IF1, IF2) exposed at both the sidesof the gate electrode GE may be retreated. The surface of the insulatingfilm (IF1, IF2) exposed thereat may be thus retreated. However, it ispreferable that the upper insulating film IF2 remains when the etchingis ended. For example, when a silicon oxide film is used as the gateinsulating film GI and when the lower insulating film (silicon oxidefilm) IF2 is exposed, the lower insulating film IF2 is rapidly etchedand the lower barrier layer BA is exposed to etching atmosphere, whichmay cause the barrier layer BA to be damaged by etching. Accordingly, itis preferable to stop the etching in the state where the upperinsulating film remains. The preferred thickness of the insulating film(IF1, IF2), for functioning as an an etching stopper, can be changeddepending on etching conditions and the type of the insulating film;however, it is preferable in the present embodiment to cause the totalthickness of the insulating films IF1 and IF2 to be, for example,approximately 0.03 to 0.1 μm. Additionally, it is preferable to causethe thickness of the upper insulating film IF2 to be approximately 0.02to 0.07 μm in order to make the insulating film IF2 remain as much aspossible when the etching is ended. Additionally, it is preferable tocause the thickness of the insulating film IF1 to be approximately 0.01to 0.03 μm, in consideration that the later-described tunnel phenomenonis likely to be produced.

The gate electrode GE is patterned to have a shape protruding in onedirection (to the right side, i.e., to the drain electrode DE side inFIG. 8). In other words, the patterning is performed such that a fieldplate electrode is provided as part of the gate electrodes GE. The fieldplate electrode is a partial region of the gate electrode GE, and refersto an electrode portion extending from the end portion of the trench Tnear to the drain electrode DE toward the drain electrode DE. The gateelectrode GE also protrudes in another direction (to the left side,i.e., to the source electrode SE side in FIG. 8). However, the amount ofthe protrusion to the drain electrode DE side is larger than that to thesource electrode SE side.

Subsequently, the interlayer insulating film IL1 is formed over the gateelectrode GE, as illustrated in FIG. 9. For example, a silicon oxidefilm is deposited, as the interlayer insulating film IL1, over the gateelectrode GE and the insulating film (IF1, IF2) so as to have athickness of approximately 0.7 μm, by using a CVD method, or the like.

Subsequently, the contact holes C1S and C1D are formed in the interlayerinsulating film IL1 by using a photolithography technique and an etchingtechnique, as illustrated in FIG. 10. By etching the interlayerinsulating film IL1 in the region where the source electrode SE isformed with the use, for example, of a non-illustrated photoresist filmas a mask, the contact hole C1S is formed, and by etching the interlayerinsulating film IL1 in the region where the drain electrode DE isformed, the contact hole C1D is formed. In the etching, the insulatingfilm (IF1, IF2) located below the interlayer insulating film IL1 is alsoremoved. Thereby, the barrier layer BA is exposed from the bottom ofeach of the contact holes C1S and C1D. Each of the contact holes C1S andC1D is thus arranged over the barrier layer BA on both the sides of thegate electrode GE. The contact hole (C1G) is also formed over the gateelectrode GE, when the contact holes C1S and C1D are formed (see FIG.2).

Subsequently, a conductive film CL is formed over the interlayerinsulating film IL1 including the insides of the contact holes C1S andC1D, etc., as illustrated in FIG. 11. The ohmic layer is first formedover the interlayer insulating film IL1 including the insides of thecontact holes C1S and C1D. For example, a titanium (Ti) film isdeposited over the interlayer insulating film IL1 including the insidesof the contact holes C1S and C1D so as to have a thickness ofapproximately 0.05 μm, by using a sputtering method, or the like.Subsequently, an aluminum film is deposited, as a metal film, over theohmic layer so as to have a thickness of approximately 0.6 μm, by usinga sputtering method, or the like. Subsequently, a heat treatment isperformed in order to reduce the coupling resistance between the barrierlayer BA and the ohmic layer. The heat treatment is performed, forexample, under nitrogen atmosphere at 650° C. for approximately 30seconds. Alternatively, an aluminum alloy, other than aluminum, may beused as the metal film. For example, an alloy of Al and Si (Al—Si), thatof Al and Cu (copper) (Al—Cu), and that of Al, Si, and Cu (Al—Si—Cu),etc., can be used as the aluminum alloy.

Subsequently, the source electrode SE and the drain electrode DE areformed in and over the contact holes C1S and C1D by patterning the Ti/Alfilm with the use of a photolithography technique and an etchingtechnique, as illustrated in FIG. 12. A photoresist film PR4, coveringthe region where the source electrode SE is formed and the region wherethe drain electrode DE is formed, is formed over the conductive film CLby using, for example, a photolithography technique, so that theconductive film CL is etched by using the photoresist film PR4 as amask. Thereby, the source electrode SE and the drain electrode DE areformed. In this case, the conductive film CL is also embedded in thecontact hole C1G over the gate electrode GE, and the gate wiring GL isformed thereover (see FIG. 2). Thereafter, the photoresist film PR4 isremoved.

Subsequently, the protective insulating film (also referred to as asurface protective film) PRO is formed over the insulating film IL1including over the source electrode SE and the drain electrode DE, asillustrated in FIG. 13. For example, a silicon oxynitride (SiON) film isdeposited, as the protective insulating film PRO, over the interlayerinsulating film IL1 including over the source electrode SE and the drainelectrode DE by using a CVD method, or the like.

The semiconductor device illustrated in FIG. 1 can be formed by theabove steps. Herein, the above steps are one example, and thesemiconductor device according to the present embodiment may bemanufactured by the steps other than the above steps.

In the present embodiment, a charge (herein, electron) can beaccumulated in the upper insulating film IF2 by forming the upperinsulating film IF2 with a film having a bandgap smaller than that ofthe lower insulating film IF1, as described above, thereby allowing theelectric field strength at a corner of the trench to be improved. As aresult, a channel is fully formed even at a corner of the trench,thereby allowing an ON-resistance to be reduced and an ON-current to beincreased. Thus, the driving capability of the transistor can beimproved.

FIGS. 14A to 14D are schematic views of the vicinity of an end portionof the bottom surface of the trench, these views presented forexplaining effects in the present embodiment. When a single-layerinsulating film (e.g., silicon oxide film) IF is used as the cover film,the clearance between the equipotential curves located between thebottom surface of the insulating film IF and that of the gate electrodeGE becomes large, as illustrated in FIG. 14A. On the other hand, when acharge (herein, electron) is accumulated in the upper insulating filmIF2, as in the present embodiment, the clearance between theequipotential curves located between the bottom surface of theinsulating film IF2 and that of the gate electrode GE becomes small, asillustrated in FIG. 14B. An ON-current can be made large by thusenhancing the electric field at a source end, thereby allowing thedriving capability of the transistor to be improved.

In particular, the thickness of the gate insulating film GI becomeslarge at an end portion (corner) of the bottom surface of the trench T,thereby possibly producing a situation in which the voltage applied tothe gate electrode GE is relaxed and a channel is less likely to beformed. Additionally, when the thickness of the gate insulating film GIis so large as to be larger than the depth of the trench T (the firstdepth, the second depth), as illustrated in FIG. 14C, a situation isproduced in which a channel is further less likely to be formed. Forexample, when the thickness of the gate insulating film GI is so largeas to be larger than the depth of the trench T (the first depth, thesecond depth), as illustrated in FIG. 14C, the thickness of the gateinsulating film GI at an end portion (corner) of the bottom surface ofthe trench T is likely to be relatively large, as compared to the casewhere the thickness thereof is so small as to be smaller than the depthof the trench T, as illustrated in FIG. 14D, thereby possibly producinga situation in which a channel is further less likely to be formed.Herein, CP represents a current pathway.

When the transistor is intended to be driven, for example, by a voltageof approximately 20 V or more in consideration of the withstand voltageof the transistor, the thickness of the gate insulating film GI isrequired to be 0.1 μm (1000 A) or more, as described above, even when aninsulating material having a good withstand voltage (e.g., aluminumoxide or silicon oxide) is selected. On the other hand, if the depth ofthe trench T is made too large, there is the fear that a failure asdescribed below may be produced. If the depth of the trench T is madetoo large, the etching of the trench T becomes difficult. Additionally,if the channel layer CH is dug deeply, the ratio, at which the currentpathway (see CP in FIG. 14C) is occupied by the sidewall of the trench Twhen the transistor is operating, becomes large, thereby allowing theresistance to be increased. Accordingly, it is preferable that thebottom of the trench T is located at a depth of approximately 0.01 to0.02 μm (approximately 0.01 μm in the present embodiment) from theboundary (two-dimensional electron gas 2DEG) between the barrier layerBA and the channel layer CH. It is possible to cause both the thicknessof the barrier layer BA and the depth of the trench T to be large; inthis case, however, it is difficult to ohmic-couple the source electrodeSE to the barrier layer BA and the drain electrode DE to the barrierlayer BA, thereby causing the resistances between them to be increased.Additionally, in order to form the barrier layer (AlGaN) BA so as tohave a good crystalline property, it is preferable to adjust thethickness thereof within a range of 0.02 to 0.04 μm (200 to 400 A).

As described above, there is the tendency that the thickness of the gateinsulating film GI becomes large and the depth of the trench T becomessmall, which makes it more important to tackle the aforementionedproblem that a channel is less likely to be formed.

On the other hand, a charge (herein, electron) can be accumulated in theupper insulating film IF2 by forming the upper insulating film IF2 witha film having a bandgap smaller than that of the lower insulating filmIF1 according to the present embodiment, thereby allowing the electricfield strength at a corner of the trench to be improved. Thereby, theelectric field at a source end can also be enhanced, so that a channelcan be formed effectively.

Subsequently, the reason why a charge (herein, electron) is accumulatedin the upper insulating film IF2, and a method of accumulating a chargewill be described with reference to FIGS. 15A to 16. FIGS. 15A to 15Dare band diagrams illustrating situations of charge injection. FIG. 16is a timing chart illustrating both a step of injecting a charge intothe upper insulating film in the transistor and a driving step of thetransistor.

Injection of a charge into the upper insulating film IF2 can beperformed by providing, to the gate electrode GE, a high voltage(potential V1) of such a degree that a tunnel current flows through thelower insulating film IF1. For example, when the insulating film IF1,which has been described in the present embodiment and has a thicknessof approximately 0.02 μm, is used, a charge can be injected into theupper insulating film IF2 from the two-dimensional electron gas 2DEG viathe insulating film IF1 by applying a potential of approximately 30 to50 V to the gate electrode GE. Alternatively, when the thickness of theinsulating film IF1 is approximately 0.01 to 0.02 μm, a charge can beinjected into the insulating film IF2 with a potential of approximately30 to 50 V.

When the potential to be applied to the gate electrode GE is small(e.g., approximately 10 V), a tunnel phenomenon is not produced, asillustrated in FIG. 15A. On the other hand, when a high potential (e.g.,approximately 40 V) is applied to the gate electrode GE, a tunnelphenomenon is produced and a charge is injected into the upperinsulating film IF2 via the insulating film IF1, as illustrated in FIG.15B. Because the bandgap of the upper insulating film (SiN) IF2 issmaller than that of the lower insulating film (SiO₂) IF1, thetunnel-injected electron (e⁻) is accumulated in the upper insulatingfilm (SiN) IF2. Further, the bandgap of the gate insulating film (Al₂O₃)is larger than that of the upper insulating film (SiN) IF2, the injectedelectron (e⁻) is held in the upper insulating film (SiN) IF2. Whencharges are fully injected as illustrated in FIG. 15C, the potentialdifference between the arrows becomes small due to the injection ofcharges. The injected charges are held also in the state where apotential is not being applied to the gate electrode GE. Additionally,when the transistor is operating, that is, when the transistor is causedto be in a conduction state by applying the threshold potential (e.g.,approximately 10 V) to the gate electrode, a tunnel phenomenon is notproduced as illustrated in FIG. 15D, and hence a new charge is notinjected and the injected electrons (e⁻) are still held in the upperinsulating film (SiN) IF2, thereby causing no trouble in an ON operationof the transistor.

For example, the potential (electron injection potential) V1 is appliedto the gate electrode GE for a period t1 in a stand-by period St, asillustrated in FIG. 16. In this state, each of a source potential and adrain potential is, for example, 0 V. Thereby, a charge is accumulatedin the upper insulating film IF2. Thereafter, the transistor is causedto be in an ON state by applying, to the gate electrode GE, thepotential (threshold potential) V2 for a period t2 in a switching periodSw during which the transistor is ON/OFF operated. In this state, thesource potential is, for example, 0 V and the drain potential is, forexample, 0 to 10 V. The potential V1 is larger than the potential V2.The potential V1 is 30 to 50 V, and the potential V2 is 5 to 15 V. Theperiod t1 is approximately 1 to 10 seconds, and the period t2 isapproximately 10⁻⁸ to 10⁻⁴ seconds.

A charge can be thus accumulated in the upper insulating film IF2, andthe clearance between the equipotential curves can be made small at asource end. Further, an ON current can be made large by enhancing theelectric field at a source end, thereby allowing the driving capabilityof the transistor to be improved.

Second Embodiment

In First Embodiment, a charge is accumulated in the upper insulatingfilm IF2 by using the gate electrode GE and by applying a high potentialto the gate electrode GE; however, a charge may also be accumulated inthe upper insulating film IF2 by providing and using a charge injectionelectrode CIE.

Hereinafter, a semiconductor device according to the present embodimentwill be described in detail with reference to the views. In theembodiment, the configurations of parts other than the charge injectionelectrode CIE are the same as those of the semiconductor deviceaccording to First Embodiment. Accordingly, the same structures andmanufacturing steps as those in First Embodiment will be brieflydescribed.

[Structure Description]

FIGS. 17 and 19 are sectional views illustrating a configuration of thesemiconductor device according to the present embodiment. FIG. 18 is aplan view illustrating the configuration of the semiconductor deviceaccording to the embodiment. The sectional view of FIG. 17 corresponds,for example, to A-A Section in FIG. 18, and the sectional view of FIG.19 corresponds, for example, to B-B Section in FIG. 18.

The semiconductor device according to the present embodiment is also aMIS-type field effect transistor using a nitride semiconductor,similarly to the First Embodiment. The semiconductor device according tothe embodiment is also a so-called recess gate-type semiconductordevice.

In the semiconductor device according to the present embodiment, achannel layer CH and a barrier layer BA are formed in this order over asubstrate S, as illustrated in FIG. 17. An active region AC, in which atransistor is to be formed, is divided by an element isolation regionISO (see FIG. 18).

An insulating film (IF1, IF2) is formed over the barrier layer BA. Thisinsulating film includes two layers of insulating films. A lowerinsulating film IF1 is formed over the barrier layer BA, and an upperinsulating film IF2 is formed over the lower insulating film IF1.

This insulating film (IF1, IF2) has a role of an etching stopper when agate electrode GE is patterned. The upper insulating film IF2 is a filmhaving a bandgap smaller than that of the lower insulating film IF1.Additionally, the upper insulating film IF2 is a film having a bandgapsmaller than that of the later-described gate insulating film GI.

In the present embodiment, a charge injection electrode CIE is furtherformed over the insulating film IF2.

The gate electrode GE is formed, via the gate insulating film GI, in atrench T that penetrates the charge injection electrode CIE, theinsulating film (IF1, IF2), and the barrier layer BA to reach the middleof the channel layer CH. The channel layer CH and the barrier layer BAinclude a nitride semiconductor, and the barrier layer BA is a nitridesemiconductor having a bandgap wider (having an electron affinitysmaller) than that of the channel layer CH. The charge injectionelectrode CIE includes a conductive film.

A two-dimensional electron gas 2DEG is generated in the vicinity of theinterface between the channel layer CH and the barrier layer BA, thevicinity being near to the channel layer CH. When a threshold potential(potential V2>0) is applied to the gate electrode GE, a channel isformed in the vicinity of the interface between the gate insulating filmGI and the channel layer CH.

The configuration of the semiconductor device according to the presentembodiment will be further described in detail. In the semiconductordevice according to the embodiment, the channel layer CH including anitride semiconductor is formed over the substrate S, and the barrierlayer BA including a nitride semiconductor is formed over the channellayer CH, as illustrated in FIG. 17. Alternatively, a nucleation layer,a strain relaxation layer, and a buffer layer, etc., may be providedbetween the substrate S and the channel layer CH in this order from thesubstrate S side.

The gate electrode GE is formed, via the gate insulating film GI, in thetrench T that penetrates the charge injection electrode CIE, theinsulating film (IF1, IF2), and the barrier layer BA and is dug to themiddle of the channel layer CH.

Each of the charge injection electrode CIE and the insulating film (IF1,IF2) has an opening in an opening region OA1 (see FIG. 21). The trench Tis formed to correspond to the opening.

The gate insulating film GI is formed in the trench T and over thecharge injection electrode CIE and the insulating film (IF1, IF2). Thegate electrode GE is formed over the gate insulating film GI. The planarshape of the gate electrode GE is, for example, rectangular (see FIG.18). Herein, the planar shapes of the gate insulating film GI and thegate electrode GE are the same as each other. The charge injectionelectrode CIE has both a rectangular portion located below the gateelectrode GE and a contact part pulled out from this portion (alsoreferred to as a pulled-out part), as described later. In the portion ofthe trench T, however, the charge injection electrode CIE is not formed(see FIG. 29).

The gate electrode GE has a shape protruding in one direction (to theright side, i.e., to a drain electrode DE side in FIG. 1). Thisprotruding part is referred to as a field plate electrode. The gateelectrode GE also extends from an end portion of the trench T near to asource electrode SE toward the source electrode SE. The charge injectionelectrode CIE is arranged below the gate electrode part protruding(extending) toward the drain electrode DE or the source electrode SE,and the insulating film (IF1, IF2) is further arranged below the chargeinjection electrode CIE.

Additionally, the source electrode SE and the drain electrode DE areformed over the barrier layer BA located on both the sides of the gateelectrode GE. The barrier layer BA and either of the source electrode SEor the drain electrode DE is ohmic-coupled via an ohmic layer. Thesource electrode SE includes both a coupling part (plug), located in acontact hole C1S formed in an interlayer insulating film IL1, and awiring part located over the coupling part. The drain electrode DEincludes both a coupling part (plug), located in a contact hole C1Dformed in the interlayer insulating film IL1, and a wiring part locatedover the coupling part. The source electrode SE and the drain electrodeDE are covered with a protective insulating film PRO. The planar shapeof each of the source electrode SE and the drain electrode DE is, forexample, rectangular (see FIG. 18).

The aforementioned gate electrode GE is coupled to gate wiring GL via acoupling part (plug) located in a contact hole C1G formed in theinterlayer insulating film IL1 (see FIG. 18). The charge injectionelectrode CIE is pulled out from a lower portion of the gate electrodeGE to be coupled to the charge injection wiring CIL via a coupling part(plug) located in a contact hole C1CI formed in the interlayerinsulating film IL1 (see FIGS. 18 and 19).

By forming the upper insulating film IF2 with a film having a bandgapsmaller than that of the lower insulating film IF1, as described above,a charge (herein, electron) can be accumulated in the upper insulatingfilm IF2, similarly to First Embodiment, thereby allowing the electricfield strength at a corner of the trench to be improved. As a result, achannel is fully formed even at the corner of the trench, therebyallowing an ON-resistance to be reduced and an ON-current to beincreased. Thus, the driving capability of the transistor can beimproved.

Because the charge injection electrode CIE is provided over the upperinsulating film IF2, a charge can be injected into the insulating filmIF2 with a voltage lower than that in First Embodiment. Additionally,the gate electrode GE and the charge injection electrode CIE can beindividually controlled, and hence a charge injection step and a drivingstep of the transistor can be individually performed.

[Description of Manufacturing Method]

Subsequently, a manufacturing method of the semiconductor deviceaccording to the present embodiment will be described with reference toFIGS. 20 to 31, so that the configuration of the semiconductor devicewill be made clearer. FIGS. 20 to 31 are sectional views or plan viewseach illustrating a manufacturing step of the semiconductor deviceaccording to the embodiment.

As illustrated in FIG. 20, the channel layer (GaN) CH, the barrier layer(AlGaN) BA, the insulating film (silicon oxide film) IF1, and theinsulating film (silicon nitride film) IF2 are sequentially formed overthe substrate S that is the same as that in First Embodiment, in thesame way as that in First Embodiment. The bandgap of the lower siliconoxide film is 8.9 eV, while that of the upper silicon nitride film is4.5 eV. Thus, the bandgap of the upper silicon nitride film is smallerthan that of the lower silicon oxide film.

Subsequently, the charge injection electrode CIE is formed over theinsulating film IF2. For example, a tungsten (W) film is deposited, as aconductive film, over the insulating film IF2 by using a sputteringmethod, or the like. Alternatively, metals other than W, such as TiN(titanium nitride), and the compounds thereof (however, conductivecompounds) may be used as the charge injection electrode CIE.

Subsequently, a photoresist film PR1 for opening the element isolationregion is formed over charge injection electrode CIE by using aphotolithography technique. Subsequently, the element isolation regionISO is formed by doping boron (B) or nitrogen (N) with the use of thephotoresist film PR1 as a mask, similarly to First Embodiment.Thereafter, the photoresist film PR1 is removed. The region surroundedby the element isolation region ISO will serve as the active region AC(see FIG. 18).

Subsequently, a photoresist film PR2 having an opening in the openingregion OA1 is formed over the charge injection electrode CIE by using aphotolithography technique, as illustrated in FIG. 21. Subsequently, thecharge injection electrode CIE and the insulating film (IF1, IF2) areetched by using the photoresist film PR2 as a mask. A gas, such as, forexample, Cl₂ or CF₄, can be used as an etching gas for the chargeinjection electrode CIE. A gas, such as, for example, CF₄ or CHF₃, canbe used as an etching gas for a silicon nitride film and a silicon oxidefilm. Thereby, a laminated film of the charge injection electrode CIEand the insulating film (IF1, IF2), having an opening in the openingregion OA1, is formed over the barrier layer BA. In other words, thebarrier layer BA is exposed in the opening region OA1 (FIG. 21).

Continuously, the trench T, which penetrates the charge injectionelectrode CIE, the insulating film (IF1, IF2), and the barrier layer BAto reach the middle of the channel layer CH, is formed by etching thebarrier layer BA and the channel layer CH with the use of thephotoresist film PR2 as a mask, as illustrated in FIG. 22. Dry etchingis performed under plasma atmosphere by using, for example, ahalogen-based gas (Cl₂, HBr, BCl₃, or the like) as the etching gas. Forexample, ICP (Inductively Coupled Plasma), or the like, can be used asthe plasma source. When the barrier layer (AlGaN) BA having a thicknessof 0.03 μm is used, the etching is performed to the depth ofapproximately 0.04 μm from the surface of the barrier layer (AlGaN) BAin order to surely remove the two-dimensional electron gas 2DEG in theopening region OA1. Thereby, the barrier layer (AlGaN) BA and thesurface portion of the channel layer (GaN) CH in the opening region OA1are removed, so that the channel layer (GaN) CH is exposed.Subsequently, the photoresist film PR2 is removed.

Subsequently, the gate insulating film GI is formed in the trench T andover the charge injection electrode CIE, as illustrated in FIG. 23. Forexample, an aluminum oxide film (alumina, Al₂O₃) having a thickness ofapproximately 0.1 μm is deposited, as the gate insulating film GI, inthe trench T and over the insulating film (IF1, IF2) by using an ALDmethod, or the like, similarly to First Embodiment. The bandgap of thegate insulating film GI is larger than that of the insulating film(silicon nitride film) IF2 located therebelow. The bandgap of thealuminum oxide film is 6 eV, while that of the silicon nitride filmlocated therebelow is 4.5 eV.

Alternatively, the silicon oxide film described in First Embodiment or ahigh dielectric constant film having a dielectric constant higher thanthat of the silicon oxide film may be used as the gate insulating filmGI. Herein, the thickness (0.1 μm) of the gate insulating film (siliconoxide film) GI is larger than the depth (the first depth and the seconddepth) of the trench T, similarly to First Embodiment.

Subsequently, the gate electrode GE is formed over the gate insulatingfilm GI. For example, a laminated film including, for example, a nickel(Ni) film and a gold (Au) film located thereabove (also referred to asan Au/Ni film) is deposited, as a conductive film, over the gateinsulating film GI by using a sputtering method, or the like, similarlyto First Embodiment.

Subsequently, the gate electrode GE is formed by patterning the gateelectrode GE and the gate insulating film GI with the use of aphotolithography technique and an etching technique, as illustrated inFIGS. 24 to 26. A photoresist film PR3, covering the region where thegate electrode GE is formed, is formed by using, for example, aphotolithography technique, so that the gate electrode GE and the gateinsulating film GI are etched by using the photoresist film PR3 as amask. Dry etching is performed under plasma atmosphere by using, forexample, a halogen-based gas (Cl₂, HBr, BCl₃, or the like, or a mixedgas thereof) as the etching gas. For example, ICP (Inductively CoupledPlasma), or the like, can be used as the plasma source. Thereafter, thephotoresist film PR3 is removed. Thereby, the rectangular gate electrodeGE can be formed, as illustrated in FIG. 26. The charge injectionelectrode CIE is exposed around the gate electrode GE (FIGS. 24 to 26).

Subsequently, the charge injection electrode CIE is patterned by using aphotolithography technique and an etching technique, as illustrated inFIGS. 27 to 29. A photoresist film PR32, covering the region where thecharge injection electrode CIE is formed, is formed by using, forexample, a photolithography technique, so that the charge injectionelectrode CIE is etched by using the photoresist film PR32 as a mask.Dry etching is performed under plasma atmosphere by using, for example,Cl₂ or HBr as the etching gas. For example, ICP (Inductively CoupledPlasma), or the like, can be used as the plasma source. Thereafter, thephotoresist film PR32 is removed. Thereby, the charge injectionelectrode CIE, which has both a rectangular portion located below thegate electrode GE and a contact part pulled out from this portion (alsoreferred to as a pulled-out part, a protruding part on the left side ofFIG. 29), can be formed, as illustrated in FIG. 29. In the portion ofthe trench T, the charge injection electrode CIE is removed in the stepof forming the trench T. Accordingly, the hatched portion illustrated inFIG. 29 serves as the region where the charge injection electrode CIE isto be formed.

The insulating film (IF1, IF2) serves as an etching stopper when thecharge injection electrode CIE is etched. If dry etching is performed onthe charge injection electrode CIE directly formed over the barrierlayer BA without forming the insulating film (IF1, IF2) by using thephotoresist film PR32 as a mask, the barrier layer BA may be damaged andthe operating characteristic of the transistor may be deteriorated,similarly to First Embodiment.

On the other hand, the insulating film (IF1, IF2) functions as anetching stopper in the present embodiment, and hence the characteristicsof the transistor can be improved.

Additionally, the surface of the insulating film (IF1, IF2) exposed atboth the ends of the gate electrode GE may be retreated. However, it ispreferable that the upper insulating film IF2 remains when the etchingis ended. For example, when a silicon oxide film is used as the gateinsulating film GI and when the lower insulating film (silicon oxidefilm) IF2 is exposed, the lower insulating film IF2 is rapidly etchedand the lower barrier layer BA is exposed to the etching atmosphere,which may cause the barrier layer BA to be damaged by etching. Becausethe thickness of the barrier layer BA is small as described above, thetwo-dimensional electron gas is likely to be affected when the barrierlayer BA is damaged. Accordingly, it is preferable to stop etching inthe state where the upper insulating film remains.

The gate electrode GE is patterned to have a shape protruding in onedirection (to the right side, i.e., to the drain electrode DE side inFIG. 24). In other words, the patterning is performed such that a fieldplate electrode is provided as part of the gate electrodes GE. The fieldplate electrode is a partial region of the gate electrode GE, and refersto an electrode portion extending from the end portion of the trench Tnear to the drain electrode DE toward the drain electrode DE. The gateelectrode GE also protrudes in another direction (to the left side,i.e., to the source electrode SE side in FIG. 24). However, the amountof the protrusion to the drain electrode DE side is larger than that tothe source electrode SE side.

Subsequently, the interlayer insulating film IL1, the source electrodeSE, and the drain electrode DE are formed over the gate electrode GE, asillustrated in FIGS. 30 and 31. After the interlayer insulating film IL1is formed, for example, in the same way as that in First Embodiment, thecontact holes C1S and C1D are formed. In this case, the contact hole(C1G) is formed over the gate electrode GE, while the contact hole C1CIis formed over the charge injection electrode CIE (see FIGS. 18 and 31).

Subsequently, a conductive film CL is formed over the interlayerinsulating film IL1 including the insides of the contact holes C1S andC1D in the same way as that in First Embodiment, and then the sourceelectrode SE and the drain electrode DE are formed by patterning theconductive film CL. In this case, the gate wiring GL is formed in andover the contact hole C1G above the gate electrode GE, and the chargeinjection wiring CIL is formed in and over the contact hole C1CI abovethe charge injection electrode CIE (see FIGS. 18 and 31).

Subsequently, the protective insulating film PRO is formed over theinsulating film IL1 including over the source electrode SE and the drainelectrode DE in the same way as that in First Embodiment (see FIG. 17,etc.).

The semiconductor device illustrated in FIG. 17 can be formed by theabove steps. Herein, the above steps are one example, and thesemiconductor device according to the present embodiment may bemanufactured by the steps other than the above steps.

Thus, the upper insulating film IF2 is formed with a film having abandgap smaller than that of the lower insulating film IF1 also in thepresent embodiment, similarly to First Embodiment, and hence a charge(herein, electron) can be accumulated in the upper insulating film IF2,thereby allowing the electric field strength at a corner of the trenchto be improved. As a result, a channel is fully formed even at thecorner of the trench, thereby allowing an ON-resistance to be reducedand an ON-current to be increased. Thus, the driving capability of thetransistor can be improved (see FIGS. 14 and 15).

FIGS. 32A and 32B are schematic views of the vicinity of an end portionof the bottom surface of the trench, these views presented forexplaining effects in the present embodiment. FIG. 33 is a timing chartillustrating both a step of injecting a charge into the upper insulatingfilm in the transistor and a driving step of the transistor.

When a single-layer insulating film (e.g., silicon oxide film) IF isused as the cover film, the clearance between the equipotential curveslocated between the bottom surface of the insulating film IF and that ofthe gate electrode GE becomes large, as illustrated in FIG. 32A. On theother hand, when a charge (herein, electron) is accumulated in the upperinsulating film IF2, as in the present embodiment, the clearance betweenthe equipotential curves located between the bottom surface of theinsulating film IF2 and that of the gate electrode GE becomes small, asillustrated in FIG. 32B. An ON-current can be made large by thusenhancing the electric field at a source end, thereby allowing thedriving capability of the transistor to be improved.

Injection of a charge into the upper insulating film IF2 can beperformed by providing, to the charge injection electrode CIE, a highvoltage (potential V1) of such a degree that a tunnel current flowsthrough the lower insulating film IF1. For example, when the insulatingfilm IF1, which has been described in the present embodiment and has athickness of approximately 0.02 μm, is used, a charge can be injectedinto the upper insulating film IF2 from the two-dimensional electron gas2DEG via the insulating film IF1 by applying a potential ofapproximately 30 to 50 V to the charge injection electrode CIE.Alternatively, when the thickness of the insulating film IF1 isapproximately 0.01 to 0.03 μm, a charge can be injected into theinsulating film IF2 with a potential of approximately 15 to 30 V.

For example, the potential V1 is applied to the charge injectionelectrode CIE for a period t1 in a stand-by period St, as illustrated inFIG. 33. In this state, each of the source potential and the drainpotential is, for example, 0 V. Thereby, a charge is accumulated in theupper insulating film IF2. Thereafter, the transistor is caused to be inan ON state by applying, to the gate electrode GE, the potential V2 fora period t2 in a switching period Sw during which the transistor isON/OFF operated. In this state, the source potential is, for example, 0V and the drain potential is, for example, 0 to 10 V. The potential V1is 15 to 30 V, and the potential V2 is 5 to 15 V. The period t1 islonger than the period t2. The period t1 is approximately 1 to 10seconds, and the period t2 is approximately 10⁻⁸ to 10⁻⁴ seconds.

Thus, a charge can be accumulated in the upper insulating film IF2, theclearance between the equipotential curves can be made small at a sourceend, and an ON-current can be made large by enhancing the electric fieldat the source end, thereby allowing the driving capability of thetransistor to be improved.

The invention made by the present inventors has been specificallydescribed above based on preferred embodiments; however, it is needlessto say that the invention should not be limited to the preferredembodiments and various modifications may be made to the inventionwithin a range not departing from the gist of the invention.

In the aforementioned First Embodiment (see FIG. 15), an example hasbeen described, in which a silicon oxide film, a silicon nitride film,and an aluminum oxide film are used as the combination of the insulatingfilm IF1, the insulating film IF2, and the gate insulating film GI. Onthe other hand, other combinations may be used, in which the bandgaps ofthese films (Eg (IF1), Eg (IF2), and Eg (GI)) satisfy the relationships:Eg (IF1)>Eg (IF2); and Eg (GI)>Eg (IF2). For example, a combination of asilicon oxide film, a silicon nitride film, and a silicon oxide film maybe used as the combination of the insulating film IF1, the insulatingfilm IF2, and the gate insulating film GI.

Additionally, a charge is injected in the stand-by period St and thetransistor is driven (ON/OFF operated) in the subsequent switchingperiod Sw; however, the stand-by period St may be provided only once atthe initial stage, or provided regularly (every predetermined period) inthe switching period Sw. Alternatively, a charge may be injected intothe insulating film IF2 before every ON/OFF operation.

What is claimed is:
 1. A semiconductor device comprising: a firstnitride semiconductor layer formed above a substrate; a second nitridesemiconductor layer that is formed over the first nitride semiconductorlayer and has a bandgap wider than that of the first nitridesemiconductor layer; an insulating film formed above the second nitridesemiconductor layer; a trench that penetrates the insulating film andthe second nitride semiconductor layer to reach a middle of the firstnitride semiconductor layer; and a gate electrode formed in the trenchand over the insulating film via a gate insulating film, wherein theinsulating film has a first film and a second film formed over the firstfilm, wherein a bandgap of the second film is smaller than that of thefirst film, and wherein a charge is accumulated in the second film. 2.The semiconductor device according to claim 1, wherein the bandgap ofthe second film is smaller than that of the gate insulating film.
 3. Thesemiconductor device according to claim 2, wherein the first film is anoxide film and the second film is a nitride film.
 4. The semiconductordevice according to claim 3, wherein the first film is a silicon oxidefilm and the second film is a silicon nitride film.
 5. The semiconductordevice according to claim 4, wherein the gate insulating film is asilicon oxide film or an aluminum oxide film.
 6. The semiconductordevice according to claim 1, wherein a thickness of the gate insulatingfilm is larger than a level difference between a surface of the secondnitride semiconductor layer and a bottom surface of the trench.
 7. Thesemiconductor device according to claim 1, wherein the thickness of thegate insulating film is larger than a level difference between a surfaceof the second film and the bottom surface of the trench.
 8. Thesemiconductor device according to claim 1, wherein the charge isinjected into the second film by applying a first potential to the gateelectrode.
 9. The semiconductor device according to claim 8, wherein achannel is formed in a bottom of the trench by applying a secondpotential to the gate electrode, and wherein the first potential islarger than the second potential.
 10. The semiconductor device accordingto claim 9, wherein a period during which the first potential is beingapplied to the gate electrode is longer than a period during which thesecond potential is being applied to the gate electrode.
 11. Asemiconductor device comprising: a first nitride semiconductor layerformed above a substrate; a second nitride semiconductor layer that isformed over the first nitride semiconductor layer and has a bandgapwider than that of the first nitride semiconductor layer; an insulatingfilm formed above the second nitride semiconductor layer; a conductivefilm formed over the insulating film; a trench that penetrates theconductive film, the insulating film, and the second nitridesemiconductor layer to reach a middle of the first nitride semiconductorlayer; and a gate electrode formed in the trench and over the conductivefilm via a gate insulating film, wherein the insulating film has a firstfilm and a second film formed over the first film, wherein a bandgap ofthe second film is smaller than that of the first film, and wherein acharge is accumulated in the second film.
 12. The semiconductor deviceaccording to claim 11, wherein the bandgap of the second film is smallerthan that of the gate insulating film.
 13. The semiconductor deviceaccording to claim 12, wherein the first film is a silicon oxide film,the second film is a silicon nitride film, and wherein the gateinsulating film is a silicon oxide film or an aluminum oxide film. 14.The semiconductor device according to claim 11, wherein a thickness ofthe gate insulating film is larger than a level difference between asurface of the second nitride semiconductor layer and a bottom surfaceof the trench.
 15. The semiconductor device according to claim 11,wherein the thickness of the gate insulating film is larger than a leveldifference between a surface of the conductive film and the bottomsurface of the trench.
 16. The semiconductor device according to claim11, wherein the charge is injected into the second film by applying afirst potential to the conductive film.
 17. The semiconductor deviceaccording to claim 16, wherein a channel is formed in a bottom of thetrench by applying a second potential to the gate electrode, and whereinthe first potential is larger than the second potential.
 18. Thesemiconductor device according to claim 17, wherein a period duringwhich the first potential is being applied to the conductive film islonger than a period during which the second potential is being appliedto the gate electrode.